In the natural pivotal joint of the knee, there are three articular pairs, namely:
an inner femoro-tibial articular pair where the inner femoral condyle slides on the inner tibial plate,
an outer femoro-tibial articular pair where the outer femoral condyle slides on the outer tibial plate,
a femoro-patellar articular pair where the rear face of the patella slides on the femoral trochlea.
Knee prosthesis can be classified in various categories, depending on the articular pairs replaced:
prostheses which comprise a mechanical articulation joining the femoral and tibial components, which are referred to as constrained or semi-constrained total prostheses;
prostheses which reproduce the contact surfaces of the articular pairs, which are divided into:
prostheses involving replacement of the three articular pairs, referred to as sliding total prostheses;
prostheses involving replacement of an inner or outer femoro-tibial pair, according to the circumstances, referred to as unicompartmental prostheses; and
prostheses involving replacement of the femoro-patellar pair, referred to as patella prostheses.
It will be noted moreover that, if the two inner and outer femoro-tibial pairs are to be replaced, the tibial component may be a single one. The present invention relates only to the tibial component of a unicompartmental prosthesis.
The tibial components of unicompartmental prostheses co-operate with a metal condylar component in the form of a crescent with a surface which substantially matches a torus and which is disposed on the prepared condyle by screwing or sealing a lug with an acrylic cement into a cavity provided in the condyle.
The current tibial components of unicompartmental prostheses are composed of a generally flat metal base portion in the form of a segment of a circular disc, with a lower face bearing grooves intended to facilitate engagement of an acrylic cement. A plate of polymer material, typically a high density polyethylene, is provided on the base portion by being moulded thereon. That plate member comprises a flat lateral face which extends a corresponding face of the base portion, extending along the chord of the segment of the circle. After the component has been set in position, the above-mentioned lateral faces bear against a wall surface which is cut into the side of the intercondylar eminence which is disposed between the two tibial plates and to which the crossed ligaments of the knee are attached. As indicated above, preparation of the tibia involves resection of the plate by means of two sawing planes, one being perpendicular to the general direction of the tibia and the other parallel to that direction, extending along the intercondylar eminence, to provide the above-mentioned wall surface.
The component is fixed in the notch caused by resection of the plate, by a cement, generally of acrylic type, the lower face of the base portion resting flat on the sawing plane which is perpendicular to the general direction of the tibia while the lateral face of the component bears against the wall surface cut in the side of the intercondylar eminence.
That type of tibial component suffers from the conventional disadvantages of cemented prostheses, namely the risk of ageing of the polymerised cement which can then crumble away or crack, as well as the risks of deterioration of the bone in contact with the aged acrylic cement, the result being that the tibial component suffers from play. In addition, when setting the prosthesis in position, it is necessary to avoid using an excess of cement in the region adjacent the angle between the sawing planes, such excess being insufficiently squeezed out when the component is forced into the notch and giving rise to the danger of causing vertical mal-alignment of the plate member.
Moreover, if wear of the plate member of the prosthesis requires replacement of the plate member, the entire component has to be removed, the notch surfaces of the tibia have to be re-cut and a new component which is thicker than the first component has to be set in place thereon. Now, the upper epiphysis of the tibia is of an only limited volume, and it is unfortunate to cut into it too deeply.
Tibial components of unicompartmental prostheses which are to be set in position without cement have been proposed. Such prostheses are formed solely by a plate member of polymer material. The plate member is provided with a cylindrical leg with staggered circular fins separated by grooves. The legs can be driven with a force fit into bores provided in the tibial epiphysis in the central part of the sawing plane which is perpendicular to the general direction of the tibia. That type of component also suffers from disadvantages. Setting the component in position requires the sacrifice of a not inconsiderable part of the epiphysis, which makes future replacement of the component a risky proposition. Furthermore, the bore into which the leg of the component is forced is provided in a region where the bony tissue is more in a spongy state than in the corticoidal state and therefore relatively soft, which in not very favourable to a durable hold.